How to Estimate Cordless Tool Battery Runtime from Amp-Hours and Workload

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A battery’s amp-hour rating is useful, but it is not a direct promise of working time. A 5.0Ah pack will not necessarily run twice as long as a 2.5Ah pack in every tool, and a saw cutting hardwood behaves very differently from an impact driver driving small screws. A practical estimate starts with the battery’s watt-hours, then adjusts for the tool’s workload and the time the motor is actually running.

Convert amp-hours to watt-hours

Amp-hours measure electrical charge. Watt-hours measure stored energy, which makes them more useful for comparing battery packs across voltage platforms.

Watt-hours (Wh) = nominal battery voltage (V) × amp-hours (Ah)

For example, a nominal 18V 5.0Ah battery stores approximately 90Wh:

18V × 5.0Ah = 90Wh

A 20V MAX battery is usually a 20V-class pack with a nominal voltage around 18V. That means a 20V MAX 5.0Ah battery and an 18V 5.0Ah battery typically contain about the same amount of energy. Marketing voltage should not be used to assume one has more runtime.

Use power draw for a first runtime estimate

The simplest runtime formula is:

Runtime in hours = battery watt-hours ÷ tool power draw in watts

If a tool averages 300 watts and the battery contains 90Wh, the theoretical runtime is 0.3 hours, or 18 minutes. This assumes the battery can deliver all of its rated energy and the tool draws a constant 300 watts. Real work rarely matches those conditions.

Battery packs lose some usable capacity because of heat, voltage drop, electronic cutoffs, and conversion losses. A reasonable planning figure is 80% to 90% of the nameplate energy for a healthy pack. Using the same example:

90Wh × 0.85 ÷ 300W = 0.255 hours

That equals about 15 minutes of continuous motor operation. If you are choosing a replacement or second battery, 18V 5Ah cordless tool batteries are often the practical middle ground between weight, price, and capacity.

Account for duty cycle and workload

Most cordless tools are not running continuously. An impact driver may run for two seconds, stop while you position the next fastener, then run again. A reciprocating saw may run for 20 seconds at a time. A grinder can remain loaded for several minutes without a break. Runtime should therefore be separated into motor-on time and elapsed job time.

Suppose a drill averages 250 watts while drilling, but the motor runs only 25% of the time. A 90Wh battery with 85% usable energy provides:

90Wh × 0.85 ÷ 250W = 0.306 hours of motor-on time

That is roughly 18 minutes of trigger time. At a 25% duty cycle, it could cover around 72 minutes of elapsed work, assuming the battery does not overheat and the drilling load stays similar.

Duty cycle changes with the job. Small pilot holes in softwood may use little power. Large holes in wet lumber, long screws, thick steel, or masonry can multiply power demand. A tool that normally lasts 30 minutes may drain a battery in 10 to 15 minutes under continuous heavy load.

Compare practical runtime by tool type

Brushless tools generally use energy more efficiently and maintain performance better as voltage falls, but they are not magic. A brushless grinder under heavy load can still empty a battery quickly. Tool size also matters: a compact drill may use less energy than a high-torque hammer drill, even when both accept the same battery.

Choose capacity without ignoring weight

Larger batteries provide more runtime, but they add cost and weight. A 8.0Ah or 12.0Ah pack can make a circular saw or rotary hammer more productive, while making an impact driver awkward for overhead work. The extra capacity is also wasted if the job consists of a few dozen fasteners.

For light assembly, trim work, and occasional repairs, a 2.0Ah or 3.0Ah pack is often the cheaper sensible choice. For a general workshop, 4.0Ah or 5.0Ah packs offer a good balance. For grinders, high-power saws, blowers, and rotary hammers, consider high-capacity cordless tool batteries, particularly if changing packs interrupts the job.

Two smaller batteries may be better than one huge battery. They let one pack charge while the other works, reduce hand fatigue, and can cost less. The trade-off is more battery changes and potentially slower charging if the charger handles only one pack at a time.

Account for the common failure points

Runtime estimates fail when the battery is cold, old, overheated, or being pushed beyond its intended output. Cold cells temporarily lose capacity. A pack that is several years old may deliver noticeably less energy than when new. High-current tools can also trigger low-voltage protection before every watt-hour is usable.

Dull blades and bits are another major cause of poor runtime. They increase cutting resistance, heat, and motor load. For a saw, replacing a dull blade can improve productivity more cheaply than buying a larger battery. Similarly, forcing a drill in a low gear or using the wrong bit can turn a reasonable runtime into a very short one.

When planning a job, calculate from watt-hours, assume about 85% usable energy, then reduce the result again if the work is unusually heavy. If the estimate says one battery will barely finish, buy or borrow a second compatible pack rather than relying on the optimistic number printed on the label. A cordless tool battery and charger kit can be good value when entering an ecosystem, but check charger speed, pack weight, and compatibility before choosing capacity alone.